Correct option is (a) Immunosuppressant drugs
To explain I would say: Our body produces a certain type of antibodies in response to the antigens. When an organ is transplanted, the body recognises the ‘non-self’ cells and starts attacking them. Therefore, Organ Transplantation rejection can be prevented by giving Immunosuppressant drugs so that the immune system gets suppressed and do not attack the ‘non-self’ cells.